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Diodes
About Diodes are the basic building blocks for most semiconductor devices. They are part of the bipolar family of devices. Diodes consist of two semiconductor materials, one doped with a deficiency of electrons called p-type material, and the other side is doped with an extra abundancy of electrons called n-type material. A process known as doping the material is what makes some parts of the diode p-type and the other parts n-type. While growing the semiconductor crystal, a small amount of arsenic or phosphorus can be mixed in with the process and dope the material. To change the type of material from p to n type, one can add some n-type chemicals to swamp out the p-type materials already in the 'substrate'. When you get done with the making of a diode, you have two types of material in direct contact with one another - this is called an abrupt pn-junction. The p-side is called the anode and collects the electrons emitted from the cathode, the n-side. Benjamin Franklin was an early experimenter with electrical devices and setup the convention for current flow. All electrical engineers around the world use his convention for current flow and it states that current flows from the positive terminal of the battery through the circuit returning back to the negative terminal. The fact is that electrons flow the opposite way. Concern is not over the direction of the electron's flow(which only makes up half of the particles participating in electricity), but the direction of the charge. Current flows in the diode from the P terminal to the N terminal, NOT the other way around (even though electrons flow the opposite way). Essentially, its the same as a one-way check valve for electricity! E. J. Mastascusa . "Diode." Bucknell University . 2008. http://www.facstaff.bucknell.edu/mastascu/eLessonsHTML/Diodes/Diode1.html Types The following are various types of diodes: *Avalanche Diode: This type of diode operates in the reverse bias, and used avalanche effect for its operation. The avalanche breakdown takes place across the entire PN junction, when the voltage drop is constant and is independent of current. Generally, the avalanche diode is used for photo-detection, wherein high levels of sensitivity can be obtained by the avalanche process. *Laser Diode: This type of diode is different from the LED type, as it produces coherent light. These diodes find their application in DVD and CD drives, laser pointers, etc. Laser diodes are more expensive than LEDs. However, they are cheaper than other forms of laser generators. Moreover, these laser diodes have limited life. *Schottky Diodes: These diodes feature lower forward voltage drop as compared to the ordinary silicon PN junction diodes. The voltage drop may be somewhere between 0.15 and 0.4 volts at low currents, as compared to the 0.6 volts for a silicon diode. In order to achieve this performance, these diodes are constructed differently from normal diodes, with metal to semiconductor contact. Schottky diodes are used in RF applications, rectifier applications and clamping diodes. *Zener diode: This type of diode provides a stable reference voltage, thus is a very useful type and is used in vast quantities. The diode runs in reverse bias, and breaks down on the arrival of a certain voltage. A stable voltage is produced, if the current through the resistor is limited. In power supplies, these diodes are widely used to provide a reference voltage. *Photodiode: Photodiodes are used to detect light and feature wide, transparent junctions. Generally, these diodes operate in reverse bias, wherein even small amounts of current flow, resulting from the light, can be detected with ease. Photodiodes can also be used to generate electricity, used as solar cells and even in photometry. *Varicap Diode or Varactor Diode: This type of diode feature a reverse bias placed upon it, which varies the width of the depletion layer as per the voltage placed across the diode. This diode acts as a capacitor and capacitor plates are formed by the extent of conduction regions and the depletion region as the insulating dielectric. By altering the bias on the diode, the width of the depletion region changes, thereby varying the capacitance. *Rectifier Diode: These diodes are used to rectify alternating power inputs in power supplies. They can rectify current levels that range from an amp upwards. If low voltage drops are required, then Schottky diodes can be used, however, generally these diodes are PN junction diodes. *Small signal or Small current diode - These diodes assumes that the operating point is not affected because the signal is small *Large signal diodes - The operating point in these diodes get affected as the signal is large. Transient voltage supression diodes - This diode is used to protect the electronics that are sensitive against voltage spikes. *Gold doped diodes - These diodes use gold as the dopant and can operate at signal frequencies even if the forward voltage drop increases. *Super barrier diodes - These are also called as the rectifier diodes. This diodes have the property of low reverse leakage current as that of normal p-n junction diode and low forward voltage drop as that of Schottky diode with surge handling ability. *Point contact diodes - The construction of this diode is simpler and are used in analog applications and as a detector in radio receivers. This diode is built of n – type semiconductor and few conducting metals placed to be in contact with the semiconductor. Some metals move from towards the semiconductor to form small region of p- tpye semiconductor near the contact. *Peltier diodes - This diode is used as heat engine and sensor for thermoelectric cooling. *Gunn diode - This diode is made of materials like GaAs or InP that exhibit a negative differential resistance region. *Crystal diode - These are a type of point contact diodes which are also called as Cat’s whisker diode. This didoe comprises of a thin sharpened metal wire which is pressed against the semiconducting crystal. The metal wire is the anode and the semconducting crystal is the cathode. These diodes are obsolete. *Avalanche diode - This diode conducts in reverse bias condition where the reverse bias volage applied across the p-n junction creates a wave of ionization leading to the flow of large current. These didoes are designed to breakdown at specific reverse voltage in order to avoid any damage. *Vaccum diodes - This diode is two electrode vacuum tube which can tolerate high inverse voltages. Diodes are used widely in the electronics industry, right from electronics design to production, to repair. Besides the above mentioned types of diodes, the other diodes are PIN diode, point contact diode, signal diode, step recovery diode, tunnel diode and gold doped diodes. The type of diode to transfer electric current depends on the type and amount of transmission, as well as on specific applications.dhruvil patel. "All types of diodes." Accessdate: 1/2/16. http://www.instructables.com/id/Types-of-Diodes/ Category:Electronics Diode Check Today, many digital multimeter manufacturers equip their meters with a special “diode check” function which displays the actual forward voltage drop of the diode in volts, rather than a “resistance” figure in ohms. These meters work by forcing a small current through the diode and measuring the voltage dropped between the two test leads. First, connect the test leads of the meter positive (+) and negative (-) to the positive and negative terminals on the diode. In the forward bias position the voltage reading obtained will be the “normal” voltage drop of 0.5 to 0.7 volts for silicon and 0.3 volts for germanium diodes. Also, you will hear a beep sound. Next, reverse the leads and the component is now in the reverse bias position and the meter will read infinite resistance. So far, this is the normal operation of a good diode component. If the component does not change when you reverse leads then it is a faulty shorted or open component. Voltage drop The voltage drop across the diode is generally quoted at 0.5 to 0.75V, but that number depends upon the current though the diode and the temperature of the diode. For our purposes of discussion at this point, 0.6V is good starting point and the value of current is now a function of the battery voltage, Vbatt, and the resistor value, R. I = (Vbatt - 0.6)/R The resistor is used to limit the current in the diode. The 1N4001 for example should be limited to carry no more than 1 Ampere of current. At this high current value, the 1N4001 will exhibit about 1.1 V drop (not 0.6V). Diodes are extremely useful devices since they allow current to flow in one direction but not the other. In this example, an expensive electronic device is protected from inadvertently plugging its batteries in backwards. The current can only flow through the diode when the battery is hooked up with the proper polarity. In practice, this diode would have an unacceptably high voltage drop for a portable application, but MOSFETs with high-current can be used to perform this function since the MOSFET also inherently has a diode built into its back-gate which can be used to start up the circuit ... once running, the circuit can enhance the gate of the MOSFET and reduce its forward drop to a small fraction of the voltage that would run across a diode. Reverse bias In this circuit, the diode is reverse biased, i.e. OFF and it does not conduct ... unless the voltage across the battery is so high that the diode breaks down in the reverse mode. Zener diodes are built to break down at a controlled voltage in the reverse mode; diodes are built to stand-off voltage, the simplest usually is good for 50V, the 1N4001 diode is an example. The 1N4002 is good for 100V; 1N4003 200V; 1N4004 400V; 1N4005 600V; 1N4006 600V; 1N4007 1000V. The application of a reverse voltage to the p-n junction will cause a transient current to flow as both electrons and holes are pulled away from the junction. When the potential formed by the widened depletion layer equals the applied voltage, the current will cease except for the small thermal current. Forward bias When the diode or the battery is hooked up in the opposite connection (with the anode side connected to the plus terminal of the battery and the cathode side connected to the negative side of the battery), current will flow through both the diode and the resistor equally and return back to the battery. Forward biasing the p-n junction drives holes to the junction from the p-type material and electrons to the junction from the n-type material. At the junction the electrons and holes combine so that a continuous current can be maintained. 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